Effect of the Capillary Fringe on Steady-State Water Tables in Drained Lands

Abstract

When the flow in the capillary fringe above the water table is taken into account in land-drainage theory, water-table heights are lower than those predicted when incident rainfall on the surface is assumed to travel vertically through the vadose zone. An analytical solution is given here to the steady-state drainage problem of the flow of surface-incident rainfall to cylindrical drain channels for the situation of a completely tension-saturated soil above the water table. This gives the maximum effect that the unsaturated soil region above the water table can have on the water-table height for the given drainage system. Calculated results show that the water-table height above drain level is smaller for deeper drains below the soil surface and for larger drain radii than is the case when the effect of the capillary fringe is ignored. It follows that drainage design based on customary land-drainage theory ignoring the effect of a capillary fringe gives an overestimate for the drain spacing.